To prevent a dried paint "skin" from forming in your can of paint while you're off touching up a wall, lay down a thin sheet of plastic wrap, touching the paint surface. If you've never had to paint a wall or room, this tip will keep you from having to stir paint to smooth it out every time you need the can. Experienced painters likely know it already though.

This is most useful if you know you'll only use part of a can, or you'll use the can to fill a roller tray or smaller can that you'll take with you as you do your painting. While the larger can sits half-full, leave the plastic wrap in contact with the surface (but don't push it under) to keep the top of the paint from drying and forming a skin.

If you use a sheet of plastic wrap large enough to extend up the sides of the can and out around the lid, you don't have to reach into the can to fish it out when you need more paint: just lift it off from around the sides. Photo by Tokyofoodcast.